1. Field of the Invention
The present invention relates to a radar beam scanning method, on-vehicle radar apparatus and radar scanning computer program, for detecting a target object which reflects an electromagnetic waves radiated from the frequency-modulated continuous wave (FMCW) radar apparatus mounted on a automotive vehicle.
2. Description of the Related Art
On-vehicle radar apparatuses for detecting a distance, relative speed and direction of such a target around the vehicle as an obstacle or precedent vehicle have been widely employed in order to control a crash prevention and tracking travelling.
The radar apparatus of this kind is fixed at the vehicle front in such a manner that the electromagnetic wave beam for detecting the target is parallel to the road surface. Further, the radar beam is preferably collimated along the vertical direction, thereby suppressing a reflection from the road surface.
However, when the radar beam is over-collimated, the radar beam is apt to miss the target due to a vertical inclination during an acceleration & deceleration and during carrying a heavy load. Thus, the conventional on-vehicle radar have a disadvantage that the target is missed or the reflection from the target is decreased, thereby remarkably lowering a detectable distance and detection accuracy.
The detectable distance and detection accuracy are further lowered by a vertical inclination of a radar antenna due to an installation accuracy and change with the passage of time.
The disadvantage of the conventional on-vehicle radar may be overcome, as disclosed in JP4-276582A, 1992, by controlling the tilt angle of the antenna on a rotating stage in such a manner that the tilt angle is made parallel to the road surface on the basis of the road surface reflection or on the basis of the vertical inclination due to the acceleration utilizing a detected acceleration and conversion table stored beforehand of acceleration and tilt angle.
Further, it is disclosed in JP No. 3186366 that the tilt angle is adjusted by a screw in such a manner that a beam intensity reflected from the target is displayed in order to maximize the reflected intensity during the adjustment by the screw.
However, JP4-276582A has a disadvantage that the tilt angle can not always be rotated to a prescribed angle, because the road surface reflection depend upon the road surface state, ground topology and distance between the vehicle and the road surface from which the radar beam is reflected.
Further, when the tilt angle is to be controlled on the basis of the conversion table of acceleration and tilt angle, the tilt angle can not always be rotated to a prescribed angle, due to an incorrect recognition of the horizontal reference direction, or due to a shift along the vertical direction (caused by an unexpected factor other than the acceleration) of the relative position between the target and antenna.
Furthermore, JP No. 3186366 has a disadvantage that the tilt angle can not be adjusted in response to the directional change during the vehicle's travelling, although it can be easily adjusted during the installation and maintenance of the antenna.